Liquid crystal display device

ABSTRACT

An LCD device includes lower and upper substrates facing each other, a liquid crystal layer between the lower and upper substrates, a first polarizing plate on the upper substrate, a second polarizing plate below the lower substrate, and a backlight unit below the second polarizing plate. The second polarizing plate has a light-diffusion layer that is proximate to the backlight unit. The light-diffusion layer has projections that project towards and contact the backlight unit. These projections are smooth curves that do not damage the surface of the backlight unit. The total of the Haze formed by the first and second polarizing plates is at least 40%.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display (LCD) device,and more particularly, an LCD device having a polarizing plate forimproving luminance and preventing backlight Mura phenomenon.

2. Discussion of the Related Art

With development of the present information society, the demand forvarious display devices has increased dramatically quite recently.Accordingly, much effort has been expended to research and developvarious flat display devices, such as liquid crystal display (LCD),plasma display panel (PDP), electroluminescent display (ELD), and vacuumfluorescent display (VFD) devices. Some of these flat display deviceshave already been used in displays of various devices.

Among the various flat display devices, the liquid crystal display (LCD)device has been most widely used due to its numerous advantages. LCDdevices are thin, lightweight, and have a relatively low powerconsumption compared with the other types of displays, most notablyCathode Ray Tubes (CRT). This allows the LCD to substitute for the CRTin most devices. In addition to LCDs incorporated in mobile devices suchas being used as a display for a notebook computer or personal dataassistant (PDA), LCD devices have been developed for stationaryelectronic devices such as computer monitors and televisions to receiveand display broadcasting signals.

Despite various technical developments in the LCD technology withapplications in different fields, research in enhancing the picturequality of the LCD device has been in some respects lacking as comparedto other features and advantages of the LCD device. In order to use theLCD device in various fields as a general display, the key to developingthe LCD device lies on whether the LCD device can implement a highquality picture, such as high resolution and high luminance with alarge-sized screen while still maintaining lightness in weight,thinness, and low power consumption.

The LCD device includes an LCD panel for displaying a picture image, anda driving part for applying a driving signal to the LCD panel. The LCDpanel includes lower and upper glass substrates bonded to each other ata predetermined interval, and a liquid crystal layer injected betweenthe lower and upper glass substrates. At this time, the liquid crystallayer is driven according to an electric field between the lower andupper substrates, thereby controlling light transmittance. As a result,the picture image is displayed on the LCD panel.

Hereinafter, a related art LCD device will be described with referenceto the accompanying drawings. FIG. 1 is a cross-sectional viewillustrating the related art LCD device. As shown in FIG. 1, a lowersubstrate 10 of the related art LCD device includes a plurality of pixelregions (not shown) in a matrix type, and a thin film transistor (notshown) and a pixel electrode 11 formed in each pixel region. Also, anupper substrate 1 includes a color filter layer 3 for displaying variouscolors, and a common electrode 5. Then, a liquid crystal layer 13 isformed between the lower and upper substrates 10 and 1. Subsequently,first and second polarizing plates 14 a and 14 b are respectively formedon the upper substrate 1 and under the lower substrate 10 for linearlypolarizing visible light, and a backlight unit 15 is formed under thesecond polarizing plate 14 b.

Although not shown, a plurality of gate lines are formed on the lowersubstrate (TFT array substrate) 10 at fixed intervals, and a pluralityof data lines are formed perpendicular to the gate lines at fixedintervals, thereby defining the plurality of pixel regions. Then, theplurality of pixel electrodes 11 are respectively formed in the pixelregions as the matrix type, and the plurality of thin film transistorsare switchable in response to signals of the respective gate lines fortransmitting signals of the respective data lines to the respectivepixel electrodes 11. After that, a first alignment layer 12 is formed todetermine an alignment direction of liquid crystal. Also, the uppersubstrate (color filter substrate) 1 includes a black matrix layer 2 forexcluding light from portions of the lower substrate except in the pixelregions, a Red/Green/Blue color filter layer 3 for displaying thevarious colors, the common electrode 5 on an entire surface of the uppersubstrate 1 for obtaining a picture image, and a second alignment layer6 on the common electrode 5 for determining the alignment direction ofthe liquid crystal. An overcoat layer 4 protects the color filter layer3 and flattens the upper substrate 1.

FIG. 2 is a cross-sectional view taken along line I-I of FIG. 1, whichillustrates a cross-sectional structure of the second polarizing plate14 b. Referring to FIG. 2, the second polarizing plate 14 b sequentiallyincludes a first adhesive layer 20, a first passivation layer 21, apolarizer 22, a second passivation layer 23, a second adhesive layer 24,a λ/4 phase shift plate 25, a third adhesive layer 26, a CholestericLiquid Crystal (CLC) layer 27 and a third passivation layer 28. At thistime, an upper surface of the first adhesive layer 20 is in contact tothe lower substrate 10, and a lower surface of the third passivationlayer 28 is in contact to the backlight unit 15. The first, second, andthird passivation layers 21, 23 and 28 are formed ofTri-Acetyl-Cellulose (TAC).

In order to obtain the necessary thinness and lightness of an LCD modulefor a notebook PC in the LCD device having the aforementioned structure,a light-scattering means formed on a light-guiding plate of thebacklight unit 15 is formed of three sheets. The light-scattering meansreceives the light emitted from the backlight, and uniformly scattersthe received light to an entire surface of the LCD panel.

Generally, the light-scattering means is comprised of four sheets suchas a lower light-diffusion plate, first and second prism sheets, and anupper light-diffusion plate. Recently, the light-scattering means usingthe three sheets, removing the upper light-diffusion plate to decreasethe thickness of the LCD device. Thus, the light-scattering means isformed from a lower light-diffusion plate 15 a and first and secondprism sheets 15 b. However, as compared to the light-scattering meansusing four sheets, the light-scattering means using only three sheetshas problems with a backlight Mura phenomenon (referred to as Newton'sRing or Wet-Out). According to the backlight Mura phenomenon,rainbow-spots are generated on a screen when two glass substrates comein contact with each other, thereby generating spots on the screenduring displaying of the picture image. As a result, the picture imageis not smoothly displayed on the screen.

A key quality requirement for photomasks used in fabricating the displayis the absence of Mura. Mura is caused by systematic deviations in thephotomask and can be visible as stripes. Mura compromises the imagequality of the finished display. Usually the deviations causing the Muraare very small, below a few hundred nanometers. While deviations of thatsize spread over a large area can be difficult to detect by measuring,the human eye can still see them due to its high sensitivity tosystematic changes in gray scale. Laser repairs are often performed tocorrect such deviations, however, such repairs are difficult, timeconsuming, and costly as they require specialized equipment.

In order to solve the problem of the backlight Mura phenomenon in therelated art LCD device as shown in FIG. 2, a diffusion process isperformed on the polarizing plate by adding beads to the third adhesivelayer 26 between the CLC layer 27 and the λ/4 phase shift plate 25. Thisdiffusion process permits the backlight Mura phenomenon to be decreased.However, the luminance of the resulting LCD is smaller than that of anLCD having a polarizing plate in which the diffusion process is notperformed. Also, if laser repair is to be performed, it is extremelydifficult to focus on the layer of the LCD panel to be repaired whenwatching the lower substrate 10 under the microscope because of thelarge density of beads added to the third adhesive layer 26 for thediffusion process.

Specifically, one of polarizing plate characteristics, Haze, indicatesthe light-scattering intensity of transmitted light and reflected light.If the Haze value is small, brightness of the screen is greatly changedat portions such as the black matrix layer (i.e. the area excluding thelight), so that it is hard to smoothly display the picture image.Meanwhile, if the Haze value is large, deterioration in the resolutionratio results. If the diffusion process is not performed on thepolarizing plate, the Haze of the standard polarizing plate is about 0%and the luminance is about 30%. These values are such that thepolarizing plate is susceptible to the backlight Mura phenomenon.However, if the conventional diffusion process is performed on the thirdadhesive layer, the Haze of the polarizing plate is about 80% and theluminance is only about 20%. While such a Haze prevents the backlightMura phenomenon, this is an unacceptable decrease in luminance. Inpracticality, a Haze of at least 40% permits sufficient scattering toreduce the backlight Mura phenomenon to a negligible amount.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to an LCD device having apolarizing plate that substantially obviates one or more problems due tolimitations and disadvantages of the related art.

The present invention provides an LCD device having improved luminanceand substantially preventing backlight Mura phenomenon.

Additional advantages and features of the invention will be set forth inpart in the description which follows and in part will become apparentto those having ordinary skill in the art upon examination of thefollowing or may be learned from practice of the invention. Otheradvantages of the invention may be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

To achieve these and other advantages and in accordance with theinvention, as embodied and broadly described herein, in one embodiment,a liquid crystal display polarizing plate comprises a light-diffusionlayer on one surface. The liquid crystal display may have a sufficientamount of Haze to substantially eliminate backlight Mura phenomenon. Thelight-diffusion layer may have a surface including a plurality ofprojections. In this case, the projections may have round shapes and/orsmooth curves.

In another embodiment, a polarizing plate for a liquid crystal display(LCD) device comprises a light-diffusion layer, a first passivationlayer above the light-diffusion layer, a Cholesteric Liquid Crystal(CLC) layer on the first passivation layer, a first adhesive layer onthe CLC layer, a λ/4 phase shift plate on the first adhesive layer, asecond adhesive layer on the λ/4 phase shift plate, a second passivationlayer on the second adhesive layer, a polarizer on the secondpassivation layer, a third passivation layer on the polarizer, and athird adhesive layer on the third passivation layer.

The light-diffusion layer may contact a surface of the first passivationlayer. In this case, the light-diffusion layer may have a surfaceincluding a plurality of projections. These projections may have roundshapes and/or smooth curves.

In another embodiment, a liquid crystal display (LCD) device compriseslower and upper substrates facing each other, a liquid crystal layerbetween the lower and upper substrates, a first polarizing plate on theupper substrate, a second polarizing plate below the lower substrate,the second polarizing plate having a light-diffusion layer therebelow,and a backlight unit below the second polarizing plate.

The second polarizing plate may comprise a first adhesive layer, a firstpassivation layer, a polarizer, a second passivation layer, a secondadhesive layer, a λ/4 phase shift plate, a third adhesive layer, aCholesteric Liquid Crystal (CLC) layer, a third passivation layer, andthe light-diffusion layer in order of proximity to the lower substrate.

The light-diffusion layer may contact a surface of the third passivationlayer. A plurality of projections may be formed on one surface of thelight-diffusion layer. These projections may have round shapes and/orsmooth curves. The adhesive layers may be devoid of added beads. Thelight-diffusion layer may produce an amount of Haze and a density of theprojections may be less than a density of beads that would have to beadded to one of the adhesive layers to obtain the same amount of Haze

A total of Haze of the first polarizing plate and Haze of the secondpolarizing plate may be at least about 40%.

The backlight unit may comprise a light-scattering means. Thelight-scattering means may comprise a light-diffusion plate, a firstprism sheet below the light-diffusion plate, and a second prism sheetbelow the first prism sheet.

The light-diffusion layer may be adjacent to and may further contact thebacklight unit. If projections contact the backlight unit, theprojections may have shapes that do not substantially damage thebacklight unit.

In another embodiment, a method of fabricating a liquid crystal display(LCD) device comprises obtaining a first polarizing plate having alight-diffusion layer on a surface thereof and placing the polarizingplate between a lower substrate and a backlight unit of the LCD device.

The obtaining the first polarizing plate may comprise forming the firstpolarizing plate. The method may further comprise forming a plurality ofprojections on the surface of the first polarizing plate. Theseprojections may have round shapes and/or smooth curves on the surface ofthe first polarizing plate.

The method may further comprise disposing a second polarizing plate moredistal to the backlight unit than the lower substrate, wherein a totalof Haze of the first polarizing plate and Haze of the second polarizingplate is at least about 40%.

The method may further comprise disposing the light-diffusion layeradjacent to the backlight unit. The light-diffusion layer may furthercontact the backlight unit.

The method may further comprise forming a passivation layerincorporating the light-diffusion layer.

The method may further comprise light-diffusion layer on a surface of apassivation layer of the first polarizing plate. In this case, themethod may further comprise forming a plurality of projections on thesurface of the passivation layer. These projections may have roundshapes and/or smooth curves on the surface of the passivation layer. Themethod may also comprise disposing a second polarizing plate more distalto the backlight unit than the lower substrate, wherein a total of Hazeof the first polarizing plate and Haze of the second polarizing plate isat least about 40%.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a cross-sectional view illustrating a related art LCD device;

FIG. 2 is a cross-sectional view taken along line I-I of FIG. 1;

FIG. 3 is a cross-sectional view illustrating an LCD device according toan embodiment of the present invention; and

FIG. 4 is a cross-sectional view taken along line II-II of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to different embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

Hereinafter, an LCD device according to one embodiment of the presentinvention will be described with reference to the accompanying drawings.

FIG. 3 is a cross-sectional view illustrating an LCD device according toan embodiment of the present invention. As shown in FIG. 3, the LCDdevice includes a lower substrate 40, an upper substrate 30 and a liquidcrystal layer 43. The terms upper and lower refer to the relativeposition between to a viewer of the LCD device, with the upper substrate30 more proximate to the viewer than the lower substrate 40. The lowersubstrate 40 includes a plurality of pixel regions (not shown) formed ina matrix, and a thin film transistor (not shown) and a pixel electrode41 formed in each pixel region. Also, the upper substrate 30 includes acolor filter layer 33 for displaying various colors, and a commonelectrode 35, and the liquid crystal layer 43, which is formed betweenthe lower and upper substrates 40 and 30. In addition, first and secondpolarizing plates 44 a and 44 b are respectively formed on the uppersubstrate 30 and under the lower substrate 40. The first and secondpolarizing plates 44 a and 44 b linearly polarize visible light, eitherfrom the backlight unit 45 and a light-diffusion layer 59 is formedbelow the second polarizing plate 44 b. A backlight unit 45 is formedbelow the second polarizing plate 44 b having the light-diffusion layer59. The backlight unit 45 is a three-layer structure that comprises alower light-diffusion plate 45 a and first and second prism sheets 45 b.As seen in FIG. 3, the light-diffusion layer 59 is formed on a surfaceof the second polarizing plate 44 b most proximate to the backlight unit45.

Although not shown, a plurality of gate lines are formed on the lowersubstrate (TFT array substrate) 40 at fixed intervals, and a pluralityof data lines are formed perpendicular to the gate lines at fixedintervals, thereby defining the plurality of pixel regions. Then, theplurality of pixel electrodes 41 are respectively formed in the pixelregions in a matrix-type arrangement, and the plurality of thin filmtransistors are switched in response to signals of the respective gatelines for transmitting signals of the respective data lines to therespective pixel electrodes 41. Next, a first alignment layer 42 isformed to determine an alignment direction of liquid crystal. Also, theupper substrate (color filter substrate) 30 includes a black matrixlayer 32 for excluding light from portions of the lower substrate exceptin the pixel regions, a Red/Green/Blue color filter layer 33 fordisplaying various colors, the common electrode 35 on an entire surfaceof the upper substrate 30 for obtaining a picture image, and a secondalignment layer 36 on the common electrode 35 for determining thealignment direction of the liquid crystal. An overcoat layer 34 protectsthe color filter layer 33 and flattens the upper substrate 44 a.

FIG. 4 is a cross-sectional view taken along line II-II of FIG. 3, whichillustrates a cross-sectional structure of the second polarizing plate44 b. Referring to FIG. 4, the second polarizing plate 44 b sequentiallyincludes a first adhesive layer 50, a first passivation layer 51, apolarizer 52, a second passivation layer 53, a second adhesive layer 54,a λ/4 phase shift plate 55 for shifting a phase of an incident light, athird adhesive layer 56, a Cholesteric Liquid Crystal (CLC) layer 57, athird passivation layer 58 and a light-diffusion layer 59. At this time,an upper surface of the first adhesive layer 50 is in contact with thelower substrate 40, and a lower surface of the light-diffusion layer 59is in contact with the backlight unit 45. The first, second, and thirdpassivation layers 51, 53 and 58 are formed of Tri-Acetyl-Cellulose(TAC). Also, the first and second passivation layers 51 and 53 protectthe polarizer 52, and the third passivation layer 58 protects the CLClayer 57. The polarizer 52 is formed of Poly-Vinyl-Alcohol (PVA), andthe CLC layer 57 re-uses the light for improving light efficiency of thebacklight unit 45. That is, the CLC layer 57 transmits one ofright-circular and left-circular polarized light, and reflects theother, and then the CLC layer 57 transmits the light reflected by areflecting plate (not shown) of the backlight unit 45. As a result, thelight received from the backlight unit 45 is completely used.

The light-diffusion layer 59 is formed through the diffusion process byadding a diffusion enhancer such as SiO₂ particles, beads or otherprojections to a lower surface of the third passivation layer 58. Thelower surface of the light-diffusion layer 59 thus has a surface thatincludes a plurality of projections. The projections are formed in roundshapes having smooth curves that extend towards and at least some ofwhich contact the backlight unit 45. These round shapes may besubstantially formed from a segment of a sphere (e.g. hemispherical),although they may be ellipsoidal, parabolic, or quonset-shaped forexample. The light-diffusion layer 59 may be formed by performing anAnti-Glare process on the third passivation layer 58. The projectionsmay be formed directly on the third passivation layer 58 or on arelatively thin layer 60 (thin compared with the third passivation layer58) that is separate from the third passivation layer 58. Thisrelatively thin layer may be used to provide additional benefits to thestructure without increasing the thickness by a substantial amount. Thedensity of these projections is substantially less than that of thebeads in the third adhesive layer 56 of the related art and thus has thecommensurate advantages described herein. The amount of Haze produced bythe light-diffusion layer 59 and that produced by an adhesive layerhaving beads may be the same even though the density of the projectionsis substantially less than the density of beads added to the adhesivelayer. The light-diffusion layer 59 may also be coated with ananti-reflection coating to reduce the amount of light reflected by theprojections.

As above, shapes other than hemispherical may be chosen to form theuneven surface of the light-diffusion layer 59. However, because theshapes may contact the light-diffusion layer 59, the uneven surface ofthe light-diffusion layer 59 may contain smooth curves to prevent theuppermost sheet (the sheet that is most proximate to the light-diffusionlayer 59, as shown the second prism sheet of the sheets 45 b) of thebacklight unit 45 from being scratched or otherwise substantiallydamaged during fabrication of the individual device, installation intothe LCD device into the electronics, or later usage of the electronics.

Moreover, the Haze of the LCD is to the total of the Haze of the firstpolarizing plate 44 a adhering to the upper substrate 30 as well as theHaze of the second polarizing plate 44 b. Thus, the Haze of both thefirst and second polarizing plates 44 a and 44 b respectively adheringto the upper and lower substrates 30 and 40 may be controlled. Thecombined Haze may be at least 40%, which provides a sufficient amount ofHaze to substantially eliminate the background Mura phenomenon. Forexample, if the Haze of the first polarizing plate 44 a is about 28%, itis possible to maintain control the Haze of the second polarizing plate44 b such that it is at least about 12% without substantiallydetrimentally affecting the luminescence of the LCD device. If the Hazeof the first polarizing plate 44 a is instead about 12%, it is possibleto maintain the Haze of the second polarizing plate 44 b such that it isat least about 28%.

In summary, a light-scattering means is formed on a light-guiding plateof the backlight unit which decreases the thickness and weight of an LCDmodule for a notebook PC or other electronics using the LCD device. Thelight-scattering means receives the light emitted from the backlight,and uniformly diffuses the received light to an entire surface of theLCD panel. The light-scattering means contains only three sheets: alight-diffusion plate, a first prism sheet, and a second prism sheet,thereby decreasing the thickness and weight of the LCD butcorrespondingly increases generation of the backlight Mura phenomenoncompared with a light-scattering means using an additionallight-diffusion plate. An Anti-Glare or similar process is performed onthe surface of a third passivation layer of the polarizing plate to forma light-diffusion layer having a surface. This surface contains smoothcurves, such as hemispheres, that are in contact with the backlightunit. Control of the density of these curves prevents the backlight Murafrom being generated by increasing the Haze of the light-diffusion layerwhile simultaneously increasing luminescence of the LCD as compared withconventional polarizers in which the light-scattering means is containedwithin the adhesive layer closest to the backlight. In addition, becauseof the lower density of the light diffusing projections, viewinginhomogeneities in the structure using a microscope or other similarvisual inspection device is easier, thereby improving laser repaircharacteristics of the LCD.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Forexample, a combination of the light-diffusion layer on the surface ofthe polarizing plate and beads in the adhesive may be used. Thus, it isintended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1. A liquid crystal display (LCD) device comprising: lower and uppersubstrates facing each other; a liquid crystal layer between the lowerand upper substrates; a first polarizing plate on the upper substrate;and a second polarizing plate below the lower substrate, the secondpolarizing plate comprising a passivation layer and a light diffusionlayer, wherein a thin layer is the only layer disposed between thepassivation layer and the light diffusion layer; and a backlight unitbelow the second polarizing plate, wherein the thin layer is thinnerthan the passivation layer.
 2. A liquid crystal display (LCD) devicecomprising: lower and upper substrates facing each other; a liquidcrystal layer between the lower and upper substrates; a first polarizingplate on the upper substrate; a second polarizing plate below the lowersubstrate, the second polarizing plate comprising a first adhesivelayer, a first passivation layer, a polarizer, a second passivationlayer, a second adhesive layer, a λ/4 phase shift plate, a thirdadhesive layer, a Cholesteric Liquid Crystal (CLC) layer, a thirdpassivation layer, and a light-diffusion layer having a plurality ofprojections on a surface thereof; and a backlight unit below the secondpolarizing plate, wherein the light-diffusion layer directly contactsthe third passivation layer, wherein the light-diffusion layer producesan amount of Haze, and a density of the projections of thelight-diffusion layer is less than a density of beads that would have tobe added to the third adhesive layer to obtain the same amount of Haze.3. The LCD device of claim 2, wherein the third adhesive layer is devoidof added beads.
 4. The LCD device of claim 2, wherein the plurality ofprojections have round shapes.
 5. The LCD device of claim 2, wherein theplurality of projections have smooth curves.
 6. The LCD device of claim2, wherein the backlight unit comprises a light-scattering means.
 7. TheLCD device of claim 6, wherein the light-scattering means comprises alight-diffusion plate, a first prism sheet above the light-diffusionplate, and a second prism sheet above the first prism sheet.
 8. The LCDdevice of claim 2, wherein a total of Haze of the first polarizing plateand Haze of the second polarizing plate is at least about 40%.
 9. TheLCD device of claim 2, wherein the light-diffusion layer is adjacent tothe backlight unit.
 10. The LCD device of claim 9, wherein no additionallayers are disposed between the light-diffusion layer and the backlightunit.
 11. The LCD device of claim 2, wherein the projections contact thebacklight unit.
 12. The LCD device of claim 2, wherein the projectionscontacting the backlight unit have shapes that do not substantiallydamage the backlight unit.